The present invention relates generally to image-data transmitters, and more particularly to an image-data transmitter which transmits a blend of binary and multilevel images to an external apparatus via a channel. The present invention is especially suitable for a facsimile apparatus (abbreviated FAX hereinafter), and an image filing apparatus.
A modified READ coding scheme (abbreviated MR hereinafter), a modified Huffman coding scheme (abbreviated MH hereinafter), and a modified MR coding scheme (abbreviated MMR hereinafter) are often used to code a binary image, such as a character image; the MH is used for a group 3 (abbreviated G3 hereinafter) FAX, and the MMR is used for a group 4 (abbreviated G4 hereinafter) FAX. In addition, a new coding scheme designed to perform a progressive build-up indication for a soft copy communication, in which a whole binary image is roughly indicated first and an image quality thereof becomes gradually improved, has been proposed recently.
On the other hand, a discrete cosine transform (abbreviated DCT hereinafter) coding scheme is often used to code a continuous multilevel image, such as a color image. As an example of a DCT coding scheme, a base-line system using a color still picture DCT coding scheme is well-known. As shown in FIG. 1, according to the system, a target image is segmented into a plurality of blocks, each consisting of n*n (typically 8*8) picture elements (abbreviated pixels hereinafter). Each block is coded in accordance with a two-dimensional (abbreviated 2D hereinafter) DCT coding scheme (in step P1). Then, the obtained n*n DCT coefficients are divided by n*n thresholds stored in a quantizing table TA (in step P2). Each of the DCT coefficients F.sub.uv (u, v=0, 1, 2, . . . , n-1) represents a spatial frequency component for each block of image data; in particular, a coefficient F.sub.00 represents a DC component of the spatial frequency proportional to an average value of n,n pixels, and another coefficients represents an AC component thereof, the coefficient becoming high as variables u and v become large. Next, a difference between a DC component of an arbitrary block and that of a block prior to the arbitrary block is calculated (in step P3), and then Huffman-coded with reference to a Huffman code table TB (in step P4). On the other hand, each AC component is zigzag-scanned as shown in FIG. 2 so as to be converted into a one-dimensional (abbreviated 1D hereinafter) series (in step P5). Then two-dimensional Huffman coding is performed, with reference to the table TB, for data generated as a result of coding a run length of zero data and a bit number of a valid coefficient (in step P6). After all the blocks are processed, a coding operation is terminated.
However, image data including a blend of a multilevel image depending on tone and a binary image depending on resolution is compression-oriented-coded in a single mode, a coding efficiency and an image quality of a restored image are much degraded.